1. Field of the Invention
The present invention relates to the housing of micromechanical structures, such as of bulk acoustic wave (BAW) filters, surface acoustic wave (SAW) filters, resonators, sensors, such as gyroscopes or actuators, such as micropumps or the same.
2. Description of the Related Art
Chips with micromechanical structures and so-called micromechanic circuits, respectively, have an increasing share of the market in high-frequency switches and frequency filters. One of the main markets for such chips with micromechanical structures is the mobile radio market. A chip with a micromechanical structure, which is also referred to as micromechanical circuit, is a semiconductor apparatus on the surface of which a micromechanical structure is implemented. Particular housing technologies are required for such circuits, wherein the housing has to establish a cavity around the micromechanical structure.
A procedure for housing a chip with a micromechanical structure common in the prior art is to insert housing elements with a cavity consisting of ceramic. These ceramic housing structures are both too expensive and too large for the current technology requirements. Typical dimensions of such ceramic housings for a chip with a micromechanical structure are at about 3 mm×3 mm×1.3 mm. With common ceramic housing technologies, these dimensions cannot be reduced any further.
Thus, an alternative process provides for bonding wafers with micromechanical structures, so-called system wafers, wherefrom the chips with micromechanical structures will then be diced, with a second wafer, the so-called cap wafer, wherein recesses and holes are etched, so that the recesses of the second wafer form cavities over the sensitive structures of the first wafer and the holes in the second wafer make the contact pads of the first wafer accessible. Thereby, the sensitive structures are protected. With this technique, housings with significantly smaller dimensions than the previously mentioned ceramic housings can be obtained. However, the relatively expensive production process which comprises sacrificial layer process steps and bond process steps, is disadvantageous.
Thus, it would be desirable to have a possibility to provide and house micromechanical structures with a cavity, respectively, which also enables small dimensions but reduces the production effort.
US 2002/0006588 A1 describes a method for producing 3D structures with continuously varying topographical properties and characteristics in photo-sensitive epoxid resists. Particularly, the same describes the possibility of obtaining 3D structures on a first main surface of a substrate by using a negative resist, namely SU-8 produced by Microchem Corp. and Sotec Microsystems SA, by exposing the negative resist through the substrate from a second main side of the substrate opposite to the first main side. Thereby, so the statement of the US 2002/0006588 A1, the problem would be solved that when exposing the negative resist from the other side, namely directly and not through the substrate, the polymerisazion of the negative resist would start at the side of the negative resist facing away from the substrate, since the light would be increasingly weakened with increasing penetration depth by the polymerization process, so that when developing the cross-linked and polymerized negative resist film, respectively; would detach from the substrate. For generating the continuously changing 3D structures, the document suggests to sample the negative resist through the substrate with a modulated light beam or to use a gray-shade mask.
In F. G. Tseng, Y. J. Chuang, W. K. Lin: A novel fabrication method of embedded micro channels employing simple UV dose control and antireflection coating, IEEE, 02/2002 the usage of a time-controlled UV exposure at thick SU-8 resists combined with an antireflex coating on the lower surface of the resist is suggested for producing a multi-layer arrangement of embedded micro-fluidic channels. The article suggests to deposit first an antireflex coating and then an SU-8 resist layer on a substrate. Then, in two exposing steps, the parts outside the desired channels are exposed, the channel walls with a high dose to cross-link them continuously, and the channel region with a lower dose, wherein a certain channel ceiling thickness results depending on the dose. An opening region in the channel region is covered in the second exposing step to not cross-link the same so that an opening in the channel ceiling results in the final developing step. Further micro-channel layers are generated in the same way, i.e. by depositing an antireflex coating and subsequent depositing of a negative resist, exposing with different dose values, depositing a next antireflex coating, etc. Then, all micro-channel layers are developed in a common developing step, by using the opening of the last produced micro-channel layer, wherein care should be taken that also the antireflex coatings between adjacent negative resist layers clear the developing path for the lower channels.